1. Field of the Invention
The present invention relates to a forming method of a gallium nitride system compound semiconductor layer. Further, the present invention relates to light emitting devices (LEDs), electronic devices, LED arrays, LED printer heads, LED printers and the like fabricated using the forming method.
2. Related Background Art
Conventionally, a GaN epitaxial layer, that is a chief material for blue light emitting diodes and lasers, has been formed on a sapphire substrate or a single crystal substrate of silicon carbide (SiC) by the metal organic chemical vapor deposition (MOCVD) method. Devices using the GaN epitaxial layer are sold on the market. However, more than a half of the cost of the device is occupied by costs of its substrate and epitaxial layer. Thus, there exists a great obstacle of cost in application of such technology to the solid-state illumination in place of light bulbs and fluorescent lamps. For example, it has been reported that its cost is about several tens to hundred times as much as the cost of the light bulb, while its goods on the market are capable of illumination with a tenth of the power consumption of a 90-W light bulb, its life time is above 50,000 hours, and its performance largely surpasses that of the light bulb.
The sapphire substrate is most generally used as a single crystal underlayer wafer for growth of the GaN epitaxial layer (see Japanese Patent No. 2,628,404). Originally, the sapphire substrate has been developed as an underlayer substrate for silicon hetero epitaxial growth for the SOI (silicon-on-insulator) structure (so-called SOS (silicon-on-sapphire)) since 1960s. A large diameter wafer, such as an 8-inch wafer, has been developed. Thus, the technology in connection with the sapphire substrate is one of the most matured wafer technologies for the hetero epitaxial growth. However, as compared with the silicon wafer, the cost of the sapphire substrate is more than ten times as much, and hence its application range is limited, similar to the GaAs wafer. In other words, the cost of the silicon wafer is lowest, its quality is high, and its diameter is large. Further, for an optimal GaN system epitaxial growth, an 8-inch sapphire substrate is not yet usable, and only a 6-inch sapphire substrate can be presently used owing to its crystal quality. Thus, the 8-inch sapphire substrate cannot be said to be optimal for the growth of a semiconductor layer with a favorable crystal quality.
Further, the GaN single crystal epitaxial growth on the silicon substrate has been researched (see Japanese Patent Laid-open No. 2006-222402). However, due to crystal defects caused by a difference in the lattice constant, though this technology is applied to electronic devices such as transistors, it is difficult to apply to light emitting devices, such as LEDs and lasers, that are more vulnerable to crystal defects.
Furthermore, U.S. Pat. No. 6,420,242 discloses the following technology. In this technology, the GaN epitaxial layer grown on the sapphire substrate is heated with short-wavelength laser light applied from the bottom surface of the sapphire substrate, only a portion close to an interface between the GaN and the substrate is fused to be decomposed into metal Ga and nitrogen gas, and a device or device region is lifted off onto a dissimilar substrate. However, since the device region is successively transferred, its productivity is a great obstacle for acquisition of a large-area substrate. Thus, this technology is not easy to put to practical use.
As described above, the technology for forming the GaN epitaxial layer on the sapphire substrate may have an issue about costs. The technology for forming the GaN epitaxial layer on the silicon substrate or the like whose lattice constant is largely different that of the GaN may have an issue about crystal defects. And, the technology for transferring the GaN epitaxial layer grown on the sapphire substrate to the dissimilar substrate may have an issue about productivity.